1. Field of the Invention (Technical Field)
The present invention relates to gas concentration measurement and more specifically to a portable detector that measures gas concentration.
2. Description of Related Art
Walking inspection surveys for finding natural gas leaks are currently performed with a surveyor moving at a normal gait while dragging a leak detector along the ground. The surveyor pauses to further investigate an area only if the instrument alarms. Often, a low ppm plume is the sole initial indication of a leak. Thus, a useful instrument for this particular application must respond within approximately one to two seconds, be sensitive to 1 ppm changes in ambient methane concentration, and not alarm falsely. Once a leak is located, an ideal instrument could measure up to pure gas levels in order to further quantify the magnitude of the leak and the urgency required for corrective action.
Typically, leak concentrations must exceed 500 ppm to be deemed worthy of being reported. Low weight, the ability to run for a full eight hour workday, and intrinsic safety are also desirable instrument characteristics for this specific application.
There is currently no single instrument that meets all of the criteria stated above. Typically, a combination of instruments is used to accomplish leak surveying. Specifically, no instrument responds rapidly, continuously draws samples, and measures from ambient methane levels (approximately 1.7 ppm) to pure gas. For example, flame ionization detectors, which are commonly used in walking area surveys to find leaking gas, typically can detect a maximum concentration of 5000 ppm. These detectors are not intrinsically safe since they use fuel mixtures composed with approximately 40% hydrogen gas and have an internal flame. For higher gas concentrations, a combustible gas indicator is typically used. These thermal conductivity sensors measure from the lower explosion limit (5% gas) to pure gas levels. They do not make continuous measurements and require hand aspiration with a squeeze bulb. OSHA has noted that combustible gas indicators give false readings in environments in which the oxygen content is below 10%. All conventional gas leak detectors are responsive to any hydrocarbon and are not selective to methane. Nonselective detection is problematic in areas where other hydrocarbons may be present.
An embodiment of the present invention meets all the requirements for natural gas leak surveying. It measures all methane levels from ambient concentration (approximately 1.7 ppm) to pure gas. One embodiment of the present invention is preferably selective to methane and detects no other hydrocarbons. In other words, other gases do not effect the measurement of methane when using this embodiment. The one second time response of this embodiment allows it to be used in rapid walking area surveys. Embodiments of this invention also use no consumables and is constructed to be intrinsically safe.